Abstract. We propose that cloud condensation nuclei (CCN) concentrations are important
for modulating ice formation of Arctic mixed-phase clouds, through
modification of the droplet size distribution. Aircraft observations from
the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate
(ARCPAC) study in northern Alaska in April 2008 allow for identification and
characterization of both aerosol and trace gas pollutants, which are then
compared with cloud microphysical properties. Consistent with previous
studies, we find that the concentration of precipitating ice particles (>400 μm)
is correlated with the concentration of large droplets (>30 μm). We are further able to link the observed microphysical conditions
to aerosol pollution, originating mainly from long range transport of
biomass burning emissions. The case studies demonstrate that polluted
mixed-phase clouds have narrower droplet size distributions and contain 1–2
orders of magnitude fewer precipitating ice particles than clean clouds at
the same temperature. This suggests an aerosol indirect effect leading to
greater cloud lifetime, greater cloud emissivity, and reduced precipitation.
This result is opposite to the glaciation indirect effect, whereby polluted
clouds are expected to precipitate more readily due to an increase in the
concentration of particles acting as ice nuclei.